This invention relates to a method of forming a spherical shell, and more particularly to a method of forming a metal blank (blanked sheet) in the form of a spherical shell suitable for manufacturing articles having an accurate surface with a gentle curvature such as parabolic antennas, roof panels of vehicles or convex mirrors.
Traditionally, parabolic antennas and roof panels of vehicles, etc., have been manufactured in a manner wherein a metal plate is pressed between male and female dies or convex and concave dies.
However, when a curved surface having a gentle curvature is formed, there is much spring back and, moreover, the distribution of the degree of spring back is irregular due to variations in the thickness of the plate which is to be pressed. It is thus difficult to obtain worked articles having a sufficient degree of accuracy.
In the case of a manufacturing apparatus for manufacturing parabolic antennas of the type which was disclosed in Japanese patent Unexamined Publication No. 218005/1984, a member for cramping the outer periphery of the blank is formed by a flat plate, and it therefore tends to be drawn inwardly by the tension in the blank acting in opposite directions. If the blank is locally drawn inwardly, the accuracy of the formed surface is reduced.
In addition, it is necessary to effect after-work on the outer peripheral flange, etc., after the spherical shell surface has been formed. There is also a risk of increasing the profile irregularity by this after-work.